In a conventional ammonia oxidation burner a mixture of NH3, O2 and N2 react at an elevated temperature and pressure over a platinum metal catalyst to nitrogen oxides. Volatilised platinum is collected by a catchment material. Typically, both the catalyst and catchment material comprise a woven or knitted gauze and accordingly several such gauzes made into a pack secured to the burner structure by clamping means. The catalyst/catchment pack is placed on a support in the burner. The most commonly used type of support is ceramic rings, Raschig rings, placed in a basket secured to the burner structure. Usually a support gauze of woven or knitted non precious metal, for instance “Megapyr”, is placed between the main support and the catalyst/catchment pack. Such conventional burners are further described in Ullmanns Encyclopaedia, Vol. 20, pages 314-317, 4th Ed. If a N2O decomposition catalyst is used in the burner, this will replace all or a part of the support material.
The ceramic rings and possible ceramic catalyst material often move away from the periphery during operation due to thermal dilatation. This movement creates a trough which often causes the gauze pack to tear. Especially around the outer edge of the gauzes the damage might be severe due to the lowered level of the Raschig rings there. It is a common observation that the trough deepens with the number of starts and stops in the plant. This tearing represents a problem both due to loss of combustion efficiency, reduced cycle time and also a hazard problem. The ammonia slipping may form ammonium nitrate and ammonium nitrite in the downstream equipment, especially in the acid condenser. Ammonium nitrate and nitrite may decompose violently.